1. Field of the Invention
The present invention relates to a thermal print head having a plurality of heating elements which are heated selectively for printing on a thermosensitive recording paper or on a recording paper through a thermosensitive ink ribbon.
2. Description of the Prior Art
An exemplary conventional thermal print head 1 is shown in FIG. 1, in which the thermal print head 1 is in contact through a recording sheet 7 with a platen 8. This thermal print head 1 comprises an alumina substrate 2, a glass glaze layer 3, a heating resistance layer 4, an electrode layer 5, and a protective layer 6 formed one over another in that order. The electrode layer 5 is formed of aluminum. In most cases, the protective layer 6 is formed of one of the following thin films.
(1) Laminated thin film of SiO.sub.2 and Ta.sub.2 O.sub.5 films
(2) Si.sub.3 N.sub.4 thin film
(3) SiC thin film
(4) Al.sub.2 O.sub.3 thin film
In FIG. 1, only a single heating element of the thermal print head 1 corresponding to a single dot is shown in a cross section. Practically, the electrode layer 5 is formed in a pattern by an etching process and the thermal print head 1 has a plurality of such heating elements.
When the thermal print head 1 of such a construction is applied to printing, a thermosensitive recording paper or a thermal transfer paper is used as the recording sheet 7, and the heating resistance layers 4 are heated selectively by selectively supplying a current through the associated electrode layers 5 to form dots at positions corresponding to the heated heating resistance layers 4. Thus, the heating resistance layers 4 are heated selectively while the platen 8 is rotated to move the recording sheet 7 relative to the thermal print head 1 to print characters on the recording sheet 7.
Recently, the printer employing the thermal print head 1 has made progressive advancement in performance including capability of high-density color printing, capability of multiplex printing and capability of graduation printing. In either printing mode, the printer must be able to operate at a high printing speed. However, the friction between the protective layer 6 and the recording sheet 7 increases with the printing speed. Moreover, in high-speed printing operation, each dot must be formed in a very short time. Accordingly, to form a clear dot in a short time, an increased voltage is applied to the heating resistance layer 4, which raises the temperature of the thermal print head 1.
Furthermore, recording sheets 7 of satisfactory quality are not necessarily used because of the situation of the user. In some cases, a recording sheet of inferior coloring sensitivity, a recording sheet having inferior surface smoothness or a special thermosensitive recording paper formed by applying a coloring material to a thick paper is used. The thermal print head 1 must be pressed against the platen 8 by a pressure substantially twice the normal pressure to obtain high-quality, clear, uniform prints such such a special recording sheet 7 is used, which further increases the friction between the protective layer 6 and the recording sheet 7, and thereby the protective layer 6 is liable to be cracked at positions corresponding to the edges of the electrode layer 5. Accordingly, the protective layer 6 must be capable of maintaining the initial performance withstanding high-speed printing and high pressure exerted thereto by the platen 8. However, the foregoing conventional protective layers (1), (2), (3) and (4) are unable to cope with various conditions resulting from high-speed printing and the use of such a special recording sheet. The disadvantages of the foregoing conventional protective layers (1) to (4) will be described hereinafter.
(1) SiO.sub.2 /Ta.sub.2 O.sub.5 laminated thin film
This laminated thin film has a low hardness and is inferior in abrasion resistance, and hence the laminated thin film is unsuitable as a protective layer for high-speed printing and printing on the special recording sheet.
(2) Si.sub.3 N.sub.4 thin film
This thin film is liable to be cracked at positions corresponding to the edges of the electrode layer 5 by stress induced by pressure exerted thereon because aluminum used ordinarily for forming the electrode layer 5 is soft. Accordingly, this thin film is unsuitable for use as the protective layer of a thermal print head which is often pressed against the platen by a high pressure for printing on a special recording sheet.
(3) SiC thin film
This thin film is chemically unstable and reacts easily with the coloring material of the recording sheet, and hence this thin film is liable to be abraded extraordinarily. Such a disadvantage is enhanced when the thermal head is heated at a high temperature. This thin film is inferior also in crack resistance. Accordingly, this thin film is unsuitable for both high-speed printing and printing on a special recording sheet.
(4) Al.sub.2 O.sub.3 thin film
This thin film is inferior in moisture resistance, and this disadvantage becomes more conspicuous with increase in the temperature of the thermal head. Accordingly, in a thermal print head employing this thin film as a protective layer, the aluminum electrode layer is liable to be subjected to electrochemical corrosion due to the corrosive action of moisture and ions contained in the recording sheet. When the electrode layer is thus corroded, the resistance of the electrode layer increases entailing omission of dots. Accordingly, this thin film is unsuitable for use as a protective layer for a thermal print head for high-speed printing.